Devices and Methods for Solder Flow Control in Three-Dimensional Microstructures

ABSTRACT

Structures, materials, and methods to control the spread of a solder material or other flowable conductive material in electronic and/or electromagnetic devices are provided.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/488,991, filed Jun. 5, 2012, which claims the benefit of priority ofU.S. Provisional Application No. 61/493,516, filed on Jun. 5, 2011, theentire contents of which applications are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to control of the flow of aconductive material in electric, electronic and/or electromagneticdevices, such as three-dimensional microstructures (e.g., waveguidestructures), and more particularly, but not exclusively, to structures,materials, and methods to control the spread of a solder material orother flowable conductive material, such as a conductive epoxy.

BACKGROUND OF THE INVENTION

With increasing demands on computational power and data transmissionbandwidth, electronic devices and microstructures incorporating suchdevices are becoming increasingly complex necessitating a greater degreeof mechanical and electrical interconnection among components. Inresponse, three-dimensional microstructures provide a variety ofadvantages in accommodating the need for increased device performance.By way of example, three-dimensional microstructures and methods fortheir manufacture are illustrated at least at U.S. Pat. Nos. 7,948,335,7,405,638, 7,148,772, 7,012,489, 7,649,432, 7,656,256, 7,755,174,7,898,356, 8,031,037 and/or U.S. Application Pub. Nos. 2010/0109819,2011/0210807, 2010/0296252, 2011/0273241, 2011/0123783, 2011/0181376and/or 2011/0181377, each of which is hereby incorporated by referencein their entirety.

A typical approach for electrically and/or mechanically interconnectingboth planar and three-dimensional microstructures is soldering. However,it may be difficult to stop solder from wicking up the length of a metalcomponent, especially in view of the complex surface morphologies whichmay be encountered in three-dimensional microstructures, andparticularly when such structures are made of or coated with metals suchas gold, silver, copper or similar metals which are capable of promotingsolder flow. For instance, the presence of a multitude of discretecomponents, mounting surfaces, interconnected chips, and so on presentsa variety of surface height changes and void spaces prone to wickingmolten solder along the surfaces of such components.

While the phenomena of adhesion of a desired solder to base metal may becalled “wetting” and lack of it as “non-wetting”, for the purposes ofthe present application the term “wicking” is defined to connote theflow (intended or unintended) of solder along the surface of parts, eventhough the physics of the flow is not one of traditional fluidic“wicking” in the sense as it occurs when a fabric contacts water.Wicking therefore in the context of the present application is thewetting of the solder, and to stop the wicking in the presentapplication refers to stopping of wetting and flow of the solder pastthe intended boundaries. A clean thin layer of gold on platinum,intended for solder reflow, may for example, continue to wet the surfaceparticularly in non-oxidizing conditions for a great distance until thesolder thickness or composition due to interdiffusion becomesunacceptable for its intended purpose.

The unintended flow of solder throughout such microstructures may causedecreased performance, uncontrolled bond lines, shorting, solderembrittlement, and other problems. In traditional planar structures suchas circuit boards control of the solder flow can be performed with apatterned solder mask. Often such materials are either selectivelyapplied or patterned, e.g., photo-patterned, or they may bemicro-sprayed. Whereas a “solder mask”, such as a patterned planardielectric coating, may be used to stop and/or control solder flow, inan open three-dimensional structure applying such a material may berelatively difficult to achieve for multiple reasons. First, theinterconnects and/or electrical junctions where devices are to beattached may be on a layer other than the surface layer, precluding theuse of dry film. Second, a complex three-dimensional structure may behard to coat and/or pattern lithographically on more than one layer.Third, it may be desirable to ensure substantially complete removal ofany existing solder mask materials as they may degrade performance suchas RF performance, because they may not be applied with sufficientaccuracy and/or quantity for many applications (e.g., microwave devices)onto such three-dimensional structures. These problems are aggravatedwhen the desired pad dimensions for a solder or conductive adhesivecontinue to shrink from squares of hundreds of microns on a side tosquares of tens of microns, as currently is the case for some microwaveand mm-wave devices and circuits such as MMICs.

In addition, three-dimensional microstructures may include coatings ofexcellent conductors and/or noble metals, such as gold, which mayaggravate a problem of solder flowing along a conductor in anuncontrolled manner. Further, solder thickness and even conductiveadhesive thickness, as well as volume, in a particular location oftenneed to be controlled as the these parameters can determine mechanicalproperties such as strength and resistance to fatigue. Maintaining thesolder's reflow over a controlled location during attach can providecompositional control of the metals in the solder system as noblemetals, diffusion barriers, and base metals tend to dissolve to varyingdegrees and therefore impact lifetime and other properties of theelectro-mechanical junctions at the points of attach. Still, solderattach may be an important technique for high strength and reliabledevice attachment. Previous approaches for three-dimensionalmicrostructures have failed to disclose how to maintain adhesion of suchcoatings particularly when the CTE match of the wettable metals and thenon-wettable layers or “wick stop” materials may be highly mismatched.Thus, there remains a need to control flow, wetting area, and/or spreadof solder material for three-dimensional micro-electric structuresincluding, for example, those incorporated herein by reference above.

SUMMARY OF THE INVENTION

In one of its aspects, the present invention provides exemplary devicesand methods in which mechanical interlocking the of the layers byoverlapping their boundaries in three-dimensional cross-section providesmechanical and positional stability despite the large change intemperature associated with reflowing a material such a solder wouldinduce, which can be on the order of 200° C. or more. In this regard,the present invention provides devices and methods in which reliance onchemical adhesion alone to keep the materials intact is not required.

For example, the present invention may provide an electronicmicrostructure having a mounting surface having at least a portionthereof configured to bond to one or more of a metallic solder and aconductive epoxy. A wick stop structure may be disposed away from themounting surface at a location on the microstructure proximate themounting surface, and may be configured to deter the flow of one or moreof the metallic solder and the conductive epoxy from the mountingsurface to a location on the microstructure beyond the location of thewick stop structure. In this regard, the wick stop structure maycomprise a material which is non-wetting to metallic solder and mayinclude a dielectric or a non-wetting metal, such as nickel. The wickstop structure may include a shelf which extends outwardly away from asurface of the microstructure at which the shelf is located and/or mayhave a portion disposed within the microstructure. Additionally oralternatively, the shelf may circumscribe a portion or all of themicrostructure.

In a further aspect, the present invention may provide a method offorming an electronic microstructure comprising depositing a pluralityof layers over a substrate, where the layers comprise one or more of ametal material, a sacrificial photoresist material, and a dielectricmaterial, thereby forming an electronic microstructure above thesubstrate. The microstructure may include a mounting surface having atleast a portion thereof configured to bond to one or more of a metallicsolder and a conductive epoxy, and a wick stop structure disposed awayfrom the mounting surface at a location on the microstructure proximatethe mounting surface. The wick stop structure may be configured to deterthe flow of one or more of the metallic solder and the conductive epoxyfrom the mounting surface to a location on the microstructure beyond thelocation of the wick stop structure. The microstructure may be removefrom the substrate to provide a freestanding part.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following detailed description of theexemplary embodiments of the present invention may be further understoodwhen read in conjunction with the appended drawings, in which:

FIGS. 1 and 1B schematically illustrate in side cross-section a portionof a microstructure and a substrate or second structure, pre- andpost-soldering, respectively, demonstrating problematic solder flow tounintended microstructure locations;

FIGS. 2A-2C schematically illustrate a top view and side cross-sectionalviews, respectively, of exemplary wick stop structures in accordancewith the present invention;

FIGS. 3A-3C schematically illustrate side cross-sectional views offurther exemplary wick stop structures in accordance with the presentinvention;

FIGS. 4A and 4B schematically illustrate top views and sidecross-sectional views of exemplary three-dimensional microstructuresincorporating wick stop structures in accordance with the presentinvention;

FIGS. 5A and 5B schematically illustrate the microstructure of FIG. 4B,pre- and post-soldering, respectively, demonstrating containment ofsolder flow in accordance with an aspect of the present invention;

FIG. 6 schematically illustrates a side cross-sectional view of afurther exemplary three-dimensional microstructure incorporating wickstop structures and solder gap-setting structures in accordance with thepresent invention;

FIGS. 7A and 7B schematically illustrate exemplary three-dimensionalmicrostructure junctions incorporating wick stop structures and soldergap-setting structures in accordance with the present invention;

FIGS. 8A and 8B schematically illustrate isometric views in partialcross-section of a coax to ground-signal-ground (G-S-G) microstructurein accordance with the present invention;

FIGS. 8C and 8D schematically illustrate a side view and frontelevational view in partial cross-section, respectively, of the coax toG-S-G microstructure of FIGS. 8A and 8B;

FIG. 8E schematically illustrates a bottom view of the coax to G-S-Gmicrostructure of FIGS. 8A and 8B;

FIGS. 9A and 9B schematically illustrate side cross-sectional views of aSMT 2-ended device such as a resistor, pre- and post-soldering,respectively, along with exemplary microstructures comprising wick stopsin accordance with the present invention; and

FIGS. 10A and 10B schematically illustrate side cross-sectional views,pre- and post-soldering, respectively, of exemplary multi-layer flipchip mounting microstructures comprising wick stops in accordance withan aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, wherein like elements are numbered alikethroughout, an aspect of the present invention may be understood byreference to FIGS. 1A-1B in which one of the problems addressed by thepresent invention is illustrated. FIG. 1A schematically illustrates adevice 100 prior to assembly comprising a portion of a three-dimensionalmicrostructure 20 for electrical and/or mechanical connection to a baselayer 10, such as a circuit board. The base layer 10 may include a metalcomponent, such as a circuit board trace 12, to which thethree-dimensional microstructure 20 is to be attached, and attachmentmay be effected through the use of a solder 30 disposed between amounting surface 22 of the three-dimensional microstructure 20 and thecircuit board trace 12. The microstructure may for example be comprisedof copper with a thin surface coating of nickel and gold as is commonfor an electrical conductor in microelectronics. Upon heating, thesolder 30 is melted and the three-dimensional microstructure 20 iseither already in physical contact or brought into physical contact withthe molten solder 32 to effect attachment of the three-dimensionalmicrostructure 20 to the circuit board trace 12, FIG. 1B. However, dueto the wetting properties of the molten solder 32 relative to thethree-dimensional microstructure 20, the solder 32 may flow up or alongthe surface of the three-dimensional microstructure 20 substantiallybeyond the mounting surface 22 and coat additional surfaces 24, 26, 28of the three-dimensional microstructure 20, which were not intended tobe coated with the solder 32. The coating of these additional surfaces24, 26, 28 with the metallic solder 32 has the potential to causeperformance loss through reduced conductivity, shorting, uncontrolledbond line thickness, change in composition, etc. Accordingly, in one ofits aspects the present invention prevents or deters the unwanted flowof solder onto surfaces and structures which are to remain free ofsolder.

For instance, in one of the aspects of the present invention, anon-wetting material may be used to control flow, wetting area, and/orspread of a material, such as a solder, thereby substantiallyminimizing, inhibiting and/or stopping unintentional solder flow. Asused throughout the present disclosure, a non-wetting material mayinclude an insulating material, which in turn may include a dielectricmaterial, such as a photopatternable dielectric which may form a solderflow stop. The non-wetting material may also include a secondary metal,such as a plated nickel ring instead of and/or in addition to adielectric non-wetting material, for example. Nickel may form anon-wetting oxide and/or may stop the solder flow, and may be employedwhen the required use temperature may exceed the temperature limits of adielectric and/or may be used where metal to metal bonding in themicrostructure is desired. In exemplary configurations of the presentinvention, the dielectric material may be replaced with a non-wettingmetal, for example if such use would not cause shorting betweenconductors (e.g., circumscribing a metal, but not bridging twoconductors as can be done with a dielectric). The non-wetting materialmay be permanent or non-permanent. For example, when solder wettingcontrol is of concern, a dielectric material may include a selectivelysoluble material that is able to survive the release process of asacrificial material used to make a multilayer structure, but which isthen dissolved away after release and after the passivation coatings areapplied (e.g., electroless nickel followed by electroless gold). Whenstop material is dissolved at this point, a base metal may be exposed.By applying electroless nickel again, an exposed base metal (e.g.,copper) may be made to plate selectively to gold, producing a relativelythin passivated metal ring of non-wetting metal.

The non-wetting material may be grown mechanically into the lines in amulti-layer build, for example using a multi-layer three-dimensionalbuild process and/or any other suitable process. The multilayer buildprocess may desirably include an electrodeposition process, a transferbonding process, a dispensing process, a lamination process, a solidthree-dimensional printing process, a laser-fusion of particles process,a vapor deposition process, a screen printing process, a squeegeeprocess, and/or a pick-and-place process. For example, a sequentialbuild process may include one or more material integration processesemployed to form a portion and/or substantially all of an apparatus. Thesequential build process may be accomplished through processes includingvarious combinations of: (a) metal material, sacrificial material (e.g.,photoresist), insulative material (e.g., dielectric) and/or thermallyconductive material deposition processes; (b) surface planarization; (c)photolithography; and/or (d) etching or other layer removal processes.Plating techniques may be useful, although other deposition techniquessuch as physical vapor deposition (PVD) and/or chemical vapor deposition(CVD) techniques may be employed.

The sequential build process may include disposing a plurality of layersover a substrate, which may include one or more layers of a dielectricmaterial, one or more layers of a metal material and/or one or morelayers of a resist material. In exemplary configurations, a firstmicrostructural element, such as a support member and/or a solder flowstop member, may be formed of dielectric material. The support structureand/or a solder flow stop member may include an anchoring portion, suchas an aperture extending at least partially there-through. One or morelayers may be etched by any suitable process, for example wet and/or dryetching processes.

The sequential build process may also include depositing one or morelayers of thermally conductive materials, which may be deposited at anydesired location, for example at substantially the same in-planelocation as a layer of the first microstructural element and/or secondmicrostructural element. In addition, one or more layers of thermallyconductive material may be deposited at any desired location, forexample spaced apart from one or more layers of the firstmicrostructural element and/or second microstructural element.

In conjunction with the present invention any material integrationprocess may be employed to form a part and/or all of an apparatus. Forexample, one or more of a transfer bonding, lamination, pick-and-place,deposition transfer (e.g., slurry transfer), and electroplating on orover a substrate layer (which may be mid-build of a process flow) may beemployed. A transfer bonding process may include affixing a firstmaterial to a carrier substrate, patterning a material, affixing apatterned material to a substrate, and/or releasing a carrier substrate.A lamination process may include patterning a material before and/orafter a material is laminated to a substrate layer and/or any otherdesired layer. A material may be supported by a support lattice tosuspend it before it is laminated, and then it may be laminated to alayer or the material may be selectively dispensed. The material mayinclude a layer of a material and/or a portion of an apparatus, forexample pick-and-placing one or more waveguide structures on and/or overa conductive surface.

In certain instances, adhesion between a dielectric and metal may not berequired, for example where elements may be mechanically constrained. Insuch a case, the dielectric material may remain a permanent part of astructure. For example, a non-wetting material, such as a dielectricsolder flow stop member, may circumscribe a metal element, as a sheetand/or a perforated sheet. For cases where a sheet is included, thesheet may form the base of a BGA(ball grid array)-like package, whichmay enable QFN(quad-flat no-lead)-like and/or similar structuresincluding maximized RF performance, higher complexity, small pitch, etc.

In a further example of the structures to which the present inventionmay be applied, a metal material may be deposited in an aperture of afirst microstructural element, affixing the first microstructuralelement to a second microstructural element. For example when ananchoring portion includes a re-entrant profile, a first microstructuralelement may be affixed to a second microstructural element by forming alayer of a second microstructural element on a layer of a firstmicrostructural element. Sacrificial material may be removed to form anon-solid volume, which may be occupied by a gas such as air or sulfurhexafluoride, a void, or a liquid, and/or to which a firstmicrostructural element, second microstructural element and/or thermalmember may be exposed. The non-solid volume may be filled withdielectric material, and an insulative material may be disposed betweenany one of a first microstructural element, a second microstructuralelement and/or a thermal manager.

As an illustrative use of the aforementioned materials, structures, andmethods of the present invention, a coaxial microstructure 200 isillustrated in FIGS. 2A, 2B. The coaxial microstructure 200 may includenon-wetting wick stop shelves 230, 240, 242 to circumscribe devicecomponents, such as center and outer conductors 210, 220, to inhibit,minimize, and/or stop the wetting flow of solder beyond the regionintended to be coated with solder, such as an end portion 205 of thecoaxial microstructure 200, thereby addressing problems associated withthe uncontrolled flow of solder. Since the outer conductor 220 maytypically comprise a hollow structure surrounding the center conductor210, the outer conductor 220 may be provided with both an interior wickstop shelf 242 and an exterior wick stop shelf 240 to deter or preventthe flow of solder on either side of the outer conductor 220.

The wick stop shelves 240, 242, 310 may be relatively thin, e.g., afraction of the thickness of the strata (i.e., layer thickness) in whichthe wick stop shelves 240, 242, 310 are built, FIG. 2B, 3A.Alternatively or additionally, the wick stop shelf 312 may be the fullthickness of the layer depending on, for example, device requirementsand operation, FIG. 3B. For example Layer 3 of FIG. 2B may be 5 to 500microns in thickness and may be formed from one layer containingmaterials of different heights of thicknesses or may be comprised of twosublayers, one formed to the height or thickness of wick stop shelf 240and a second continuing past that height or thickness to complete thefull thickness of Layer 3. Alternatively or additionally, the wick stop252, 262 comprising a non-wetting material, may be disposed internallyto a part 250, 260 so the wick stop 252, 262 is flush with, or recessedwithin, a surface 264 of the part 250, 260, FIG. 2C. In suchconfigurations, the non-wetting nature of the wick stop 252, 262 maydeter or stop the flow of solder without the need for a mechanical shelfwhich protrudes externally from the part 250 in which the wick stop 252,262 is incorporated. The ability to stop the flow of solder without theuse of a shelf configuration may be desirable in cases where potentialbreakage of a shelf is a concern, or where mechanical interference bythe shelf or the dielectric material should be minimized. In the casesshown in FIGS. 2B and 2C, the wick-stop 252, 262 may be mechanicallyentrenched or interlocked into the central electrically conductivestructure 250, 260 so that chemical adhesion alone is not required overthe solder reflow temperature range.

The non-wetting wick stop shelves 230, 240, 242 may be formed as part ofa multi-layer build process and/or any other suitable process. By way ofexample, the coaxial microstructure 200 may be fabricated bysequentially building Layers 1 and 2, after which the wick stop shelves230, 240, 242 may be fabricated during the portion of the sequentialbuild process which provides Layer 3. In particular, Layer 1 and Layer 2may be formed by sequential build of sacrificial material and a platedand/or grown material, such as photoresist and copper electroplating. Aphotoresist mold, e.g., sacrificial material, is illustratedsubstantially removed in FIG. 2B. In addition, seed layers may beoptionally applied after each layer, as needed. In Layer 3, for example,a seed layer may be applied to Layer 2. A dielectric may be appliedand/or patterned, and this may be followed by another seed layer asneeded. A sacrificial resist may be applied and/or planarized over apermanent dielectric. The resist may be patterned and then a conductivematerial, e.g., metal, in Layer 3 may be grown by electrodeposition, forexample. Layer 3 may be planarized via CMP, for example, and more layersmay be added as desired or needed. In one exemplary configuration, apermanent dielectric (or non-conductor or a material not wettable by thesolder) 314 may be formed in multiple layers, such as two layers 316,318, FIG. 3C. The first layer 316 may provide a mechanical anchor, andthe second layer 314, formed after Layer 3, may provide a masking regionand may also set a thickness for a solder or conductive adhesive appliedto the interior region 340 of layer 318. The anchor 316 maysubstantially minimize adhesion failure of the second layer 314 inprocessing and/or use. However, if sufficient chemical adhesion can beobtained, the anchor 316 may be eliminated. Additionally oralternatively to a multi-layer build process, a non-wetting wick stopmay be provided as a coating by atomic layer deposition, or othersuitable processes, for example, over a conductive structure 250, 260.The coating may be patterned, such as by selective etching or othersuitable technique, to provide non-wetting wick stop layer(s) on desiredpotions of the conductive structure 250, 260, such as at the locationsillustrated in FIG. 2C, for example.

In view of the teaching of FIGS. 2A-2C, 3A-3C and the exemplary wickstop shelves/layers illustrated therein, solution to the problemillustrated in FIGS. 1A, 1B in accordance with one aspect of the presentinvention is illustrated in FIGS. 4A, 4B, in which respectivethree-dimensional microstructures 420, 430 are provided with wick stops425, 432. Specifically FIG. 4A schematically illustrates a top down viewand side cross-sectional view of the three-dimensional microstructure420 which may be fabricated via a multilayer build process to compriselayers 424, 426, 428. (The arrow labeled “C” conceptually illustratesthat additional portions of the three-dimensional microstructure 420 mayextend from the portion of the microstructure 420 illustrated.)Proximate an upper surface 412 of layer 424 a wick stop 425 may beprovided at a location which circumscribes layer 426. The wick stoplayer 422 may comprise any of the exemplary materials discussed aboveand may be formed by any of the fabrication techniques discussed abovein connection with FIGS. 2A-2C, 3A-3C. In particular, the wick stop 425may include a wick stop shelf 423 extending beyond the perimeter oflayer 424 to mechanically block the flow of solder upward from layer 424onto layer 426. In addition, the wick stop 425 may include a wick stoplayer 422 overlying the upper surface 412 of the layer 424 in aconfiguration where the wick stop layer 422 prevents or deters the flowof solder due to the non-wetting nature of the material from which thewick stop layer 422 is composed.

In a further exemplary configuration in accordance with the presentinvention, FIG. 4B schematically illustrates a top down view and sidecross-sectional view of a three-dimensional microstructure 430 which maybe fabricated in a manner similar to that of microstructure 420 andwhich includes layers 434, 436, 438. (The arrow labeled “C” againconceptually illustrates that additional portions of the microstructure430 may extend beyond that which is shown in FIG. 4B.) A wick stop shelf432 may be provided near a lower surface 431 of layer 434, which maycomprise any of the exemplary materials discussed above and may beformed by any of the fabrication techniques discussed above inconnection with FIGS. 2A-2C, 3A-3C. The ability of the wick stop shelf432 to deter or prevent the flow of solder is further illustrated inFIG. 5A which shows the three-dimensional microstructure 430 in positionprior to assembly above base layer 510 to which the three-dimensionalmicrostructure 430 is to be attached. The base layer 510 may include acircuit board, hybrid circuit, semiconductor device, a furtherthree-dimensional structure, and/or any other suitable structure, andmay include a metallic component 512 disposed thereon. Solder or solderpaste 530 may be disposed intermediate the three-dimensionalmicrostructure 430 and the metallic component 512. Upon melting, thesolder 532 flows to mechanically and/or electrically connect thethree-dimensional microstructure 430 to the metallic component 512 ofthe base layer 510. However, in contrast to the situation illustrated inFIG. 1B, the wick stop shelf 432 deters or prevents the flow of solder532 upward past the wick stop shelf 432 along the surfaces of thethree-dimensional microstructure 430, FIG. 5B.

In yet a further aspect of the present invention, a three-dimensionalmicrostructure 630 may be provided with stop pads 635 which may functionto establish the thickness of a bonding material, such as a conductiveepoxy (or solder) 633, FIG. 6. The stop pads 635 may also provide fordirect or nearly direct contact between the microstructure 630 and thedevice to which is it is bonded, layer 612 on 610. Similar to thestructures depicted in FIG. 5B, FIG. 6 schematically illustratesattachment of the three-dimensional microstructure 630 to a metallicpart 612 of a base layer 610, which may include a circuit board, hybridcircuit, semiconductor device, a further three-dimensional structure,and/or any other suitable structure. Unlike the three-dimensionalmicrostructure 530 of FIG. 5B, the three-dimensional microstructure 630includes stop pads 635 disposed on a lower surface of the microstructure630 at a location intermediate microstructure 630 and the metallic part612 of the base layer 610. The stop pads 635 have a lateral height (orthickness) and, when seated in contact with the metallic part 612,provide bondline gaps 634 between the three-dimensional microstructure630 and the metallic part 612, which gaps 634 have the same height asthe stop pads 635 independent of the downward force used to attach thethree-dimensional microstructure 630 to the metallic part 612. Inaddition, the stop pads 635 may maximize or enhance wetting in thecontact region between the three-dimensional microstructure 630 and themetallic part 612. The conductive epoxy (or solder) 633 may fill thebond gaps 634 to attach the three-dimensional microstructure 630 to themetallic part 612. A wick (or flow) stop shelf 632 is also provided tocooperate with the bond gaps 634 to maintain the desired thickness ofthe conductive epoxy 633 at the periphery of the three-dimensionalmicrostructure 630, while at the same time preventing or deterring flowof the solder or conductive epoxy 633 onto portions of thethree-dimensional microstructure 630 disposed above the wick stop shelf632. It should be noted that the material of stop pads 635 could be thesame as that of other larger microstructure layers, or the non-wettableportion of 630, i.e., wick stop shelf 632, depending other aspects ofthe intended function, for example depending on the needs for theelectrical conductivity and/or its thermal conductivity, and so on. Forexample if the bulk of microstructure 630 is copper and being used forits high electrical and thermal conductivity, having mechanical stoppads 635 also composed of copper would typically provide improvedthermal and electrical conductivity between the microstructure 630 andthe substrate 610 as copper typically has better electrical and thermalconductivity than solders or conductive epoxies.

By way of further example, FIG. 7A illustrates similar principles of thepresent invention, but in the context of three-dimensionalmicrostructure junctions (i.e., exemplifying the situation where thebase layer 610 is a three-dimensional microstructure). Specifically,first and second three-dimensional microstructures 720, 740,respectively, may be provided, each of which includes respective stoppads 725, 726, 745 and a respective wick stop shelf 722, 742. The stoppads 725, 726, 745 may mechanically cooperate to set bond gaps betweenthe mating surfaces of the first and second three-dimensionalmicrostructures 720, 740 for receiving a solder (or conductive epoxy)730. Post assembly, each bond may be filled with a respective portion ofthe solder 731, 733, 735, FIG. 7B. The optional gaps shown between thestop pads 725, 726 may be used to allow excess conductive epoxy, solder,or any other bonding medium, e.g., solder 730, to flow in a controlledmanner between non-wetting layers, wick stop shelves 722, 742. Whileshelves 722, 742 are not shown mechanically constrained as are theshelves 310 in FIG. 3A, the shelves 722, 742 may readily beso-constructed.

In another of its aspects, the present invention may be utilized inconfigurations relating to coaxial to ground-signal-ground (G-S-G)launch, such as for DC-100+ GHz launches. With reference to FIGS. 8A-8E,an exemplary coaxial microstructure 800 is illustrated having an outerconductor 820 and a center conductor 810 making a downward Z-transition.The center conductor 810 may be supported within the outer conductor 820by a dielectric support 840 which may minimize upward and lateral motionof the center conductor 810 within the outer conductor 820. To providetermination of the coaxial microstructure 800 in a manner thatfacilitates mounting of the coaxial microstructure 800 to anotherdevice, such as on an MMIC or circuit board, for example, grounding feet850, 852 may be provided in electrical communication with the outerconductor, and signal foot 854 may be provided in electricalcommunication with the center conductor 810. A dielectric wick stop 830may be provided between the feet 850, 852, 854 and the center and theouter conductors 810, 820 to deter or prevent the flow of solder upwardfrom the feet 850, 852, 854 onto the center and outer conductors 810,820. In addition, the dielectric wick stop 830 may also provide supportto the center conductor 810 to provide mechanical stability of thecenter conductor 810 relative to the outer conductor 820. Alternatively,the wick stop 830 may be provided as individual wick stops on each ofthe feet 850, 852, 854 using any suitable wick stop configuration, suchas those depicted in FIGS. 2A-2C, 3A-3C, for example.

In certain configurations the coaxial microstructure 800 may serve as ajumper connecting two chips, and therefore the height of the terminalcomprising the ground and signal feet 850, 852, 854 may be within theheight of the outer conductor 820, FIG. 8C, 8D. Alternatively, if thecoaxial microstructure 800 were intended to be used in a surface mountapplication, the ground and signal feet 850, 852, 854 could be providedat a location such that the feet 850, 852, 854 are flush with thesurface of the coaxial microstructure 800 or protrude from the surfaceof the coaxial microstructure 800.

In yet a further of its aspects, the present invention may be utilizedin attach of SMT (surface-mount technology) devices or flip chipmounting applications. For example, with reference to FIGS. 9A-9B,application of the present invention to SMT mounting is illustrated inwhich microstructures such as electrical conductors 920, 940 lead tomounting pads or mounting surfaces, shown on base layer 910, such as acircuit board, ceramic substrate, IC, or lower layers ofmicrostructures. Dielectric or non-wetting wick stop shelves 922, 942may be provided proximate a surface of the mounting pads 920, 940 wheresurface mounting is to be effected, FIG. 9A. Upon application andmelting of a solder 932, 934 a surface mount component 950, such as aresistor or capacitor, may be electrically and mechanically connected tothe mounting pads 920, 940, FIG. 9B. Such solders may alternatively bealready disposed on the mounting pads 920, 940 or the surface mountcomponent's electrical mounting terminals. In such a configuration, thewick stop shelves 922, 942 deter or prevent the flow of solder downwardbeyond the mounting region onto the mounting pads 920, 940, andpotentially onto the base layer 910. In a similar manner, the presentinvention may provide advantages in flip chip mounting, includingmulti-layer applications which allow parts to be mounted over oneanother. For example, a base layer 1010, such as a circuit board orhybrid microelectronic circuit, may include mounting pads 1020, 1030,1040, 1050 of varying heights disposed thereon, FIG. 10A. Each mountingpad 1020, 1030, 1040, 1050 may include a respective dielectric wick stopshelf 1022, 1032, 1042, 1052 proximate a mounting surface of theirrespective mounting pads 1020, 1030, 1040, 1050. Chips 1060, 1070 may beflip chip mounted to the mounting pads 1020, 1030, 1040, 1050 using asolder, the flow which solder downward towards the base layer 1010 maybe deterred or prevented by the wick stop shelves 1022, 1032, 1042,1052, FIG. 10B.

These and other advantages of the present invention will be apparent tothose skilled in the art from the foregoing specification. Accordingly,it will be recognized by those skilled in the art that changes ormodifications may be made to the above- described embodiments withoutdeparting from the broad inventive concepts of the invention. It shouldtherefore be understood that this invention is not limited to theparticular embodiments described herein, but is intended to include allchanges and modifications that are within the scope and spirit of theinvention as set forth in the claims.

What is claimed is:
 1. A method of forming an electronic microstructurecomprising: depositing a plurality of layers over a substrate, whereinthe layers comprise one or more of a metal material, a sacrificialphotoresist material, and a dielectric material, thereby forming anelectronic microstructure above the substrate, the structure comprising:a mounting surface having at least a portion thereof configured to bondto one or more of a metallic solder and a conductive epoxy; and a wickstop structure disposed away from the mounting surface at a location onthe microstructure proximate the mounting surface, the wick stopstructure configured to deter the flow of one or more of the metallicsolder and the conductive epoxy from the mounting surface to a locationon the microstructure beyond the location of the wick stop structure. 2.The method according to claim 1, wherein the wick stop structurecomprises a material which is non-wetting to metallic solder.
 3. Themethod according to claim 1, wherein the wick stop structure comprises amaterial which is non-wetting to conductive epoxy.
 4. The methodaccording to claim 1, wherein the wick stop structure comprises adielectric.
 5. The method according to claim 1, wherein the wick stopstructure comprises a non-wetting metal.
 6. The method according toclaim 5, wherein the non-wetting metal comprises nickel.
 7. The methodaccording to claim 1, wherein the wick stop structure comprises a metaloxide.
 8. The method according to claim 1, wherein the wick stopstructure comprises a shelf which extends outwardly away from a surfaceof the microstructure at which the shelf is located.
 9. The methodaccording to claim 8, wherein the shelf comprises a portion disposedwithin the microstructure.
 10. The method according to claim 8, whereinthe shelf circumscribes a portion of the microstructure.
 11. The methodaccording to claim 1, wherein the wick stop structure circumscribes aportion of the microstructure.
 12. The method according to claim 1,wherein the wick stop structure is disposed internal to themicrostructure at a surface of the microstructure.
 13. The methodaccording to claim 12, wherein the wick stop structure is recessedwithin the microstructure.
 14. The method according to claim 1, whereinthe microstructure comprises a three-dimensional structure comprisingmultiple layers.
 15. The method according to claim 1, wherein themicrostructure comprises a three-dimensional structure formed by amulti-layer build process.
 16. The method according to claim 1, whereinthe mounting surface comprises a raised surface to provide a mechanicalstop between the mounting surface and a part to be mounted thereto. 17.The method according to claim 1, wherein the mounting surface comprisesmounting feet to provide a G-S-G termination.
 18. The method accordingto claim 17, wherein the wick stop structure is disposed proximate themounting feet.
 19. The method according to claim 1, wherein themicrostructure comprises a coaxial microstructure.
 20. The methodaccording to claim 1, wherein the metallic solder comprises a solderpaste.
 21. The method according to claim 1, comprising a chip flip-chipmounted to the mounting surface.
 22. The method according to claim 1,comprising a surface mount component mounted to the mounting surface.23. The method according to claim 1, comprising providing a layer ofmaterial non-wetting to one or more of metallic solder and a conductiveepoxy and selectively patterning the non-wetting layer to provide thewick stop structure.